Thanks for this tip! I created a new project in XCode 4.5.2 and is static linking against Poco and was getting similiar linking issues for all the Poco libs.So to just summarize to anyone reading this post in the future:You need to change the C++ Standard Library to the GNU oneBuild Settings- Apple LLVM compiler 4.1 - Language-- C++ Standard Library = libstdc++ (GNU C++ standard library)